If today's Google Sync announcement has convinced you that it's time to move your contact management to Google servers, you probably need to do a little housekeeping with your Gmail contacts first.
For example, if you're staring in the face of numerous duplicate contacts that should represent the same person, the built-in contact merge feature in Google Contacts is a must. Just find the duplicate contacts, tick their checkboxes, and click "Merge these contacts...." Easy peasy. To manage your contacts, either head to the Contacts page in Gmail or to the unadvertised standalone site. Thanks Michael!